This invention relates to a compact linear motor including free piston compressors (also called vibrating and linear compressors) for vapour compression systems and in particular a control system to prevent failure or damage due to unwanted changes of compression level caused by changes to ambient temperature or operating conditions.
Compressors, for example refrigerator compressors, are conventionally driven by rotary electric motors. However, even in their most efficient form, there are significant losses associated with the crank system that converts rotary motion to linear reciprocating motion. Alternatively a rotary compressor which does not require a crank can be used but again there are high centripetal loads, leading to significant frictional losses. A Linear compressor driven by a linear motor would not have these losses, and can be designed with a bearing load low enough to allow the use of aerostatic gas bearings as disclosed in U.S. Pat. No. 5,525,845.
Linear reciprocating motors obviate the need for crank mechanisms which characterise compressors powered by rotating electric motors and which produce high side forces requiring oil lubrication. Such a motor is described in U.S. Pat. No. 4,602,174. U.S. Pat. No. 4,602,174 discloses a linear motor design that is extremely efficient in terms of both reciprocating mass and electrical efficiency. This design has been used very successfully in motors and alternators that utilise the Stirling cycle. It has also been used as the motor for linear compressors. However, in the case of compressors designed for household refrigerators the design in U.S. Pat. No. 4,602,174 is somewhat larger and more costly than is desirable for this market.
The piston of a free piston compressor oscillates in conjunction with a spring as a resonant system and there are no inherent limits to the amplitude of oscillation except for collision with a stationary part, typically part of the cylinder head assembly. The piston will take up an average position and amplitude that depend on gas forces and input electrical power. Therefore for any given input electrical power, as either evaporating or condensing pressure reduces, the amplitude of oscillation increases until collision occurs. It is therefore necessary to limit the power as a function of these pressures.
It is desirable for maximum efficiency to operate free piston refrigeration compressors at the natural frequency of the mechanical system. This frequency is determined by the spring constant and mass of the mechanical system and also by the elasticity coefficient of the gas. In the case of refrigeration, the elasticity coefficient of the gas increases with both evaporating and condensing pressures. Consequently the natural frequency also increases. Therefore for best operation the frequency of the electrical system powering the compressor needs to vary to match the mechanical system frequency as it varies with operating conditions.
Methods of synchronising the electrical voltage applied to the compressor motor windings with the mechanical system frequency are well known. For a permanent magnet motor used in a free piston compressor, a back electromotive force (back EMF) is induced in the motor windings proportional to the piston velocity as shown in FIG. 8a. The equivalent circuit of the motor is shown in FIG. 8b. An alternating voltage (V) is applied in synchronism with the alternating EMF (xcex1v) in order to power the compressor. U.S. Pat. No. 4,320,448 (Okuda et al.) discloses a method whereby the timing of the applied voltage is determined by detecting the zero crossings of the motor back EMF. The application of voltage to the motor winding is controlled such that the current is zero, at the time at which the EMF intersects with the zero level to allow back EMF zero crossing detection.
Various methods have been used to limit oscillation amplitude including secondary gas spring, piston position detection, piston position calculation based on current and applied voltage (U.S. Pat. No. 5,496,153) measuring ambient and/or evaporating temperature (U.S. Pat. No. 4,179,899, U.S. Pat. No. 4,283,920). Each of these methods requires the cost of additional sensors or has some performance limitation.
It is an object of the present invention to provide a compact linear motor which goes some way to overcoming the abovementioned disadvantages or which will at least provide the public with a useful choice.
Accordingly in a first aspect the present invention may be said to consist in an electric linear motor for driving a reciprocating load comprising:
a stator having a magnetically permeable core with at least one air gap and means for producing a non constant magnetic flux in said stator and said at least one air gap;
an armature having a structure which supports at least one permanent magnet of which at least a substantial portion is located in at least one of said at least one air gap, such that the interaction of the magnetic field of said at least one permanent magnet and said non constant flux in said at least one air gap producing a force on said armature, said armature in use being connected to said load and thereby reciprocating with respect to said stator; and
energisation means for controlling said means for producing an alternating flux such that at least one end of said at least one permanent magnet passes outside the region of substantially uniform flux density present within said at least one of said at least one air gap during a portion of the reciprocal motion of said armature.
In a second aspect the present invention may be said to consist in a refrigerator which uses a compressor characterised in that the compressor and compressor motor are linear devices and said motor comprises:
a stator having a magnetically permeable core with at least one air gap and means for producing a non constant magnetic flux in said stator and said at least one air gap;
an armature having a structure which supports at least one permanent magnet of which at least a substantial portion is located in at least one of said at least one air gap, such that the interaction of the magnetic field of said at least one permanent magnet and said non constant flux in said at least one air gap producing a force on said armature, said armature in use being connected to said load and thereby reciprocating with respect to said stator; and
energisation means for controlling said means for producing an alternating flux such that at least one end of said at least one permanent magnet passes outside the region of substantially uniform flux density present within said at least one of said at least one air gap during a portion of the reciprocal motion of said armature.
In a third aspect the present invention may be said to consist in a vapour compressor comprising:
a piston,
a cylinder,
said piston being reciprocable within said cylinder, the vibrating system of piston, spring and the pressure of said vapour having a natural frequency which varies with vapour pressure,
a linear brushless DC motor drivably coupled to said piston having at least one winding,
a DC power supply,
commutation means for electronically commutating said at least one winding from said DC supply to provide a supply of current to said at least one winding to reciprocate said piston,
resonant driving means which initiate commutation of said at least one winding to thereby drive said piston at the resonant frequency of said vibrating system,
current controlling means which determine the amount of said supply of current supplied by said commutation means, said determined amount of current being related to said resonant frequency, and which initiate commutation of said at least one winding to thereby limit the amplitude of reciprocation of said piston.
In a forth aspect the present invention may be said to consist in a method for driving and controlling the amplitude of the piston in a free piston vapour compressor wherein said piston reciprocates in a cylinder and wherein the vibrating system of piston, spring and the pressure of said vapour has a resonant frequency which varies with vapour pressure, said method using a linear brushless DC motor having at least one winding and comprising the steps of:
electronically commutating said at least one winding from a DC supply to reciprocate said piston, with commutations timed to drive said piston at the resonant frequency of said vibrating system, limiting the amount of current in said at least one winding by limiting the value of a parameter which determines current supply during commutation to a value which is a function of said resonant frequency.
The xe2x80x9cevaporating temperature of the vapour entering the compressorxe2x80x9d is also referred to in this specification as the xe2x80x9cevaporator temperaturexe2x80x9d. Likewise the xe2x80x9cresonant frequencyxe2x80x9d is also referred to as the xe2x80x9cnatural frequencyxe2x80x9d.
To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.